An optical distance sensor has a light emitter for emitting a light and a light receiver for receiving the light, in which the light emitted from the emitter reaches the receiver after a reflection on a detection object. In some cases, such a distance sensor has a coaxial optical system that puts the emitter and the receiver on the same optical axis.
In case that the optical distance sensor having the coaxial optical system, the light (e.g., a laser beam or the like) emitted from the emitter passes through a front screen disposed at a front part of the sensor, and is projected toward an outside of the sensor. The light reflected by the outside object of the sensor passes through the front screen again, and enters into the light receiver.
Thus, in case that the optical distance sensor has the coaxial optical system, the light emitted from the emitter passes through the front screen, and is emitted toward the outside of the sensor. Therefore, a part of the emitted light is reflected by an inner surface of the front screen as an internal reflection. The light reflected by the inner surface of the front screen enters into the light receiver separately from the light reflected by the outside object. That is, the light caused by the internal reflection results in a false sensing in the measurement of the distance by the optical distance sensor (i.e., the internal reflection by the front screen). Thus it is desired to reduce the internal reflection as much as possible.
Further, as for an external surface of the front screen, foreign matter such as rain, soil and the like are easily deposited. When foreign matter (e.g., dirt, soil and the like) deposit on the front screen, the light emitted from the emitter is reflected by the deposited foreign matter. Therefore, when the foreign matter deposits on the front screen, the measurement accuracy of the distance to the outside object deteriorates, similar to the case when the internal reflection is caused on the inner surface of the front screen.
Thus, by disposing a triangular prism-like projecting object on the case equivalent to the front screen, and by emitting the laser light from a dirt-detection light emitter to the projecting object, the dirt is detected in a conventional sensor, (e.g., a sensor disclosed in a patent document, JP H10-090412 A (patent document 1)).
However, the sensor in the patent document 1 can detect the dirt only on a part of the case (i.e., at a position of the projecting object). Further, the sensor in the patent document 1 is required to have a separate optical system only for the detection of the dirt. Thus, the number of components is increased, and the structure of the sensor becomes complicated in the conventional sensor.